The extrathoracic airways have been neglected or excluded in many investigations of the control of breathing. It has recently become apparent that this region is important in respiratory control, both as a source of afferent information and as an effector mechanism. Receptors located in the laryngeal mucosa can, when stimulated, elicit apnea in newborn animals or complex respiratory responses in adults. Extrathoracic tracheal stretch receptors may govern the continuous regulation of expiratory airflow by several reflex pathways. The vocal cords have been shown to provide important changes in airway resistance with the respiratory cycle in awake cats. These vocal cord movements are chiefly brought about by phasic activation of the posterior cricoarytenoid muscles. In the studies proposed, upper airway physiology will be studied using several approaches. The respiratory movements of the vocal cords, which have been shown to be important in cats, will be asessed in human subjects using a fiberoptic laryngoscope. This system will be used both during quiet breathing an hyperpnea, and to study the responses of the vocal cords to single breath changes in expiratory resistance. In animal studies the role of vagal afferent feedback in determining laryngeal responses to chemoreceptor stimulation will be determined, and long-term adaptations of laryngeal muscles to increased respiratory work rates will be assessed. The chemical specificity of laryngeal receptors responsible for apnea in newborn animals will be investigated using single unanesthetized cats to investigate the role of receptors in the extrathoracic trachea and the possible importance of the upper airways in the isocapnic clearance of small concentrations of inspired CO2.